RFID sensors are used for many purposes. One of the most common types of RFID sensors is a passive RFID sensor. A passive RFID sensor includes circuitry that stores a particular encoding; a transceiver module that receives an RF signal from a reader; and circuitry that uses the energy from the received RF signal to power circuitry in the sensor that generates and transmits an RF signal that includes the stored encoding. Passive RFID sensors are popular because of the low cost of the sensor and their durability. In particular, passive RFID sensors do not require replaceable or chargeable batteries since they operate on the energy from the received RF signals.
One of the most common uses of passive RFID sensors is inventory control. To track items in a store or warehouse, passive RFID sensors with unique encodings are attached to the items in the store or warehouse. RFID readers are then placed throughout the store or warehouse. Each RFID reader periodically interrogates the RFID sensors within the coverage area of the reader to obtain the unique encodings to identify the items in the area. The interrogation is performed by transmitting an interrogation signal and reading the signals from the RFID sensors that are received by the RFID reader in predetermined time slots of a particular interrogation frame. The process is repeated for several frames until information is received from all of the RFID sensors in the coverage area of the reader.
As items tend to move within the area, it is often difficult to determine whether an RFID reader has captured the information from all of the RFID sensors in its coverage area. Currently, this is often solved by having the RFID reader perform the interrogation for a predetermined number of interrogation frames that assures that a predetermined number of RFID sensors can be interrogated. The predetermined number is often set much higher than the average number of RFID sensors that are commonly in an area to assure information from all of the RFID sensors in the area are received. However, this does not assure that all of the RFID sensors in an area have been identified. Thus, a way of estimating the number of RFID sensors in a particular area is needed to ensure that all of the items in a particular area are identified.